Automatic Train Operation (“ATO”) systems, such as GE Transportation's Incremental Train Control system (“ITCS”), typically use interlocked routes that form one dimensional rail paths. In other words, a train's path is described as a series of mile posts between starting and ending points. In contrast, rail terrain and maps are typically presented in two- or three-dimensional maps. At control points such as switches, sidings, stations, etc., the train may traverse alternate track segments depending on traffic and track resulting track availability. From a train point of view, the route it is to traverse is still along a one dimensional line. For situations where route reentry is possible, such as loops, the subsequent route and associated block occupancy is equal as previous, with a change in direction. Representation in a continuous one-dimensional system is difficult to achieve. In the case of parallel tracks with entry control points, differentiation in a one dimensional space is not possible a priori and both optional tracks would have to be interlocked. Furthermore, with most ATO systems, a continuous route is plotted prior to departure. This makes the accommodation of alternate route entries difficult.
Alternatively, the interlocking and route selection can be performed in a two- or three-dimensional space representation. This, however, requires that the location determination system on the train is capable of accurately determining location in all three dimensions on a continuous basis. In case of location determination systems such as Global Positioning System (“GPS”), altitude determination is less accurate than “X” and “Y” position determination. Additionally, in case of loss of GPS signal, the train location determination system has to revert to alternate means, such as inertial systems which are expensive, or distance calculation based on axle tachometers and the like. In the latter case, the three-dimensional location system has to transform the data to a one-dimensional system for handoff which then includes the errors in the three-to-one dimensional translation.